Normal development, growth, and function of multi-cellular organisms require control both of processes that produce cells and of those that destroy cells. Mitosis, or cell proliferation, is highly regulated except in specific states termed cell proliferative diseases. There also exist processes for destruction of cells. Cells in multi-cellular organisms die by two distinct mechanisms. One method, termed necrotic cell death, is characterized by cytoplasmic swelling, rupturing of cellular membranes, inflammation and disintegration of subcellular and nuclear components. The other method, apoptosis, by contrast, is characterized by more organized changes in morphology and molecular structure. Apoptotic cells often condense and shrink, in part, by cytoplasmic membrane blebbing, a process of shedding small packets of membrane-bound cytoplasm. The chromosomes of such cells condense around the nuclear periphery. Generally, in apoptotic cells the chromosomes are degraded by specific nucleases that cleave DNA to produce regular-sized fragments. Importantly, there is a requirement for new mRNA and protein expression during the early stages of some forms of apoptosis, indicating that it is an active process. Macrophages envelop and phagocytose apoptotic cells, thereby digesting and recycling the cellular components.
Changes in cell morphology during apoptosis are profound. Detection of the many morphological changes associated with apoptosis is detected using light microscopy or electron microscopy. In particular, electron microscopy is useful for evaluating cells with a high nucleus to cytoplasm ratio and light microscopy is useful for immuno- and histochemistry. The changes characteristic of apoptosis include decreased volume, compaction of cytoplasmic organelles, and increased cell density. In addition, microvilli disappear, blebs of cytoplasm form at the cell surface, and the blebs dissociate from the cell to form apoptotic bodies. Other techniques are useful in the analysis of apoptosis including confocal, laser, and scanning microscopy, fluorescent DNA dye binding, and molecular techniques. The molecular techniques permit detection of apoptosis in formalin-fixed and embedded tissue, including terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) and in situ, end labeling (ISEL).
Protease Involvement
The progression of apoptosis requires the coordinated action of specific proteases. The proteases can be inhibited by inhibitors including N-tosyl-L-phenylalanylchloromethyl ketone (TPCK) and N-tosyl-L-lysylchloromethyl ketone (TLCK). Furthermore, at least 10 cysteine proteases related to interleukin-1-βconverting enzyme have been identified as components of apoptotic signaling pathways. The interleukin-1-β converting enzyme-like proteases are referred to as caspases and are identified and have been isolated by molecular cloning.
In addition, there are other proteases involved in apoptosis including the granzymes and cathepsin. Granzyme B is a serine esterase that can activate several members of the caspase family. Granzyme B may be a mediator of cytotoxic T lymphocyte induced apoptosis. Granzyme B is known to cleave and initiate caspase 3, a likely component of its mode of action. Granzyme B may also initiate nuclear events associated with cytotoxic T lymphocyte-induced apoptosis, consistent with observations that it is passively transported into the nucleus and bind to nuclear proteins. One action of Granzyme B may be in the regulation of conversion of proCPP32 to CPP32. CPP32 is itself a protease thought to cleave poly(ADP-ribose) polymerase (PARP) and may also activate prolamin protease resulting in activation of lamin protease. Cleavage of lamins and inactivation of the DNA repair enzyme PARP promote the development of apoptotic changes in the cell nucleus.
Serine Proteases
In contrast to cysteine proteases, the role of serine proteases in apoptosis is controversial. For a general discussion, see Kaufmann, S. Cancer Res 1993, 53, 3976. For example, it is known that the serine protease inhibitor TLCK inhibits apoptosis-associated proteolysis. However, TLCK is known to inhibit cysteine proteases in addition to serine proteases, and has recently been shown to inhibit a member of the interleukin-1β converting enzyme family. Thus, the effect of TLCK on apoptosis is likely not mediated by an effect as a serine protease inhibitor, given the more established role of cysteine proteases in apoptosis.
Cellular Protease Targets
Multiple polypeptide species must be modified to produce the wide range of morphological manifestations that characterize apoptosis. For example, the lamins are nuclear intermediate filament proteins that form a fibrous layer between the inner nuclear membrane and the chromatin. The resulting lamina is thought to play a role in maintaining nuclear shape and in mediating chromatin-nuclear membrane interactions. Thus, the apoptosis-associated changes in nuclear shape might require lamin reorganization. Another polypeptide that is cleaved during apoptosis is poly (ADP-ribose) polymerase (PARP). PARP is an abundant nuclear enzyme that catalyzes the conversion of the dinucleotide NAD+ to nicotinamide and protein-linked chains of ADP-ribose. Yet, the detailed role of PARP in the process of apoptosis is unclear. Studies have suggested that inhibitors of PARP delay apoptosis and yet other studies have suggested that inhibition of PARP increases the fragmentation of DNA during apoptosis. It is clear, however, that PARP is proteolytically degraded late in apoptosis.
Another proteolytic enzyme target during apoptosis is the U1 ribonuclear protein (U1-70k), a molecule required for splicing of precursor mRNA that is itself cleaved to an inactive 40 kDa fragment during apoptosis. The cleavage of this polypeptide would result in cessation of RNA processing.
Other substrates for protease activity during apoptosis include fodrin, the PITSLREβ1 protein kinase, the adenomatous polyposis coli (APC) protein, the retinoblastoma gene product, terminin, and nuclear matrix proteins. Cleavage of fodrin, an abundant membrane associated cytoskeletal protein, has been detected during apoptosis in a variety of cell lines. PITSLREβ1 protein kinase, a member of the P34cdc2 gene family has been shown to induce mitotic delay in CHO cells. Members of this family appear to be cleaved during apoptosis. For example, recent studies indicate that PITSLREβ1 kinase is proteolytically cleaved during FAS- or steroid-induced apoptosis in T-cells. Another major group of protease targets is the caspases, themselves proteases, or precursor forms of caspases. Yet another group of proteins which may well be downstream effectors of caspase-mediated apoptosis, include the protein kinases PKCδ, PKCθ, MEKK1, the sterol regulatory element binding proteins 1 and 2, and the DNA fragmentation factor (DFF).
Diseases Associated with Apoptosis
Increased levels or apparent induction of apoptosis is associated with a number of diseases including cancer, autoimmune diseases including rheumatoid arthritis, neurodegenerative diseases, myocardial infarction, stroke, sepsis, ischemia-reperfusion injury, toxin induced liver injury, and AIDS (see Kidd, V. J., Annu Rev Physiol, 1998, 60, 533; List, P. J. M., et al., Arterioscler Thromb Vasc Biol 1999, 19, 14; Jabs, T., Biochem Pharmacol 1999 57, 231; Deigner, H. P., et al. Curr Med Chem 1999, 6, 399). The apoptosis appears to be mediated by oxygen free radicals [O] which have been implicated in various disorders including atherosclerosis, diabetes, sepsis, Alzheimer's disease, arthritis, muscular dystrophy, cancer, Downs syndrome, multiple sclerosis, HIV infection and other inflammatory diseases (Morel, J. B. and Dangle, J. L., Cell Death Differ 1997, 4, 671; Beal, M. F., Curr Opin Neurobiol 1996, 6, 661).